The need in many industries to make determinations based upon knowledge of the surface contour of an object is well known. Moreover it is most often necessary to make measurements to the surface contour of the object without making contact with the surface of that object, hence the term: non-contact surface contour measurement. Various conventional non-contact surface contour measurement systems which employ a scanning beam of radiation, and particularly coherent optical light as from a laser, have been described. Reference is made to U.S. Pat. No. 4,158,507 to Himmel (1979) and to U.S. Pat. No. 4,701,049 to Beckmann et al. (1987). These disclosures and the references summarized therein together with U.S. Pat. No. 4,634,879 to Penney (1987), and the references summarized therein describe the general elements of such a contour measurement system employing a scanning beam of radiation or light.
Beckmann et al. is particularly concerned with improvements related to elimination of interference by "false reflections" from points outside of the plane of the scanning beam, while Himmel describes an alternative reflection detection system for measuring relative changes in contour slope, the system employing an optical grating through which the reflections are sensed by a photomultiplier detector. Although various methods of optical triangulation of a reflected coherent light beam are disclosed and computations appear to be made which involve both the measurement of angles and of certain time related functions, none of the known systems provide a simple, highly accurate, time based triangulation measurement system which can be virtually impervious to the problem of reflection aliasing within the plane of the scanning beam itself.
In general, the phenomenon of aliasing, the occurrence of which is well known in the art, is involved wherever a reflection detection point cannot discriminate between reflections received from different scanning beam angles reflecting from differing countour heights. This problem is inherent, for example, in the measurement system proposed by Himmel (U.S. Pat. No. 4,158,507), partially illustrated in schematic in FIG. 1. Schematically illustrated optical grating 2 has typical light receiving "windows" p, q, r, s defined by adjacent light opaque grating bars. Each window is necessarily focused to view a spot of light reflected from a particular reflection point on a surface beneath it. The exact height and location (z, x) of any particular reflection point will depend on the height of the reflection surface at its intersection with the line of focus of the particular window. Thus, in FIG. 1, windows p, q, r, s are focused respectively, where the surface contour is represented by the line h=O, on points a, b, c, d. In the system proposed by Himmel, contour slope is determined by the dimension and uniformity of spacing of the points a, b, c, d. Thus, given suitable calibration, the system illustrated in FIG. 1 will read contour h=O as flat and level.
However, for an alternate contour represented in the figure by the dotted line below h=0, having reflection points a', b', c' with respective actual contour heights at those points of h.sub.1, h.sub.2, and h.sub.3, the beam angle which would have produced a reflection at point a now reflects from a', and the beam which would have reflected from point b now reflects from b', and so forth. But window q, which was aimed to receive a reflection from b at one beam angle, can also receive a reflection a' at a different beam angle. Similarly, windows r and s can each receive reflection from both c and b' and d and c', respectively.
Thus, the Himmel system cannot distinguish between contour surface h=O and the surface represented by dotted line, and consequently would read either surface as flat and level. In the illustration of FIG. 1, reflection from a' becomes an alias for reflection from b, reflection from b' becomes an alias for reflection from c, and so forth. In fact, because in the system proposed by Himmel greater resolution is achieved by greater grating densities, the amount of height difference between h=O on the one hand and h.sub.1, h.sub.2, or h.sub.3, on the other need only be very small to introduce the kind of aliasing error described above.
Accordingly it is an object of the invention to provide apparatus and methods for an improved non-contact surface contour measurement system using time based triangulation methods, the accuracy of which measurements is not diminished by the aliasing phenomenon.
Furthermore none of the known systems employ a reflected light receiving surface which acts as a light channeling surface such that reflected light is received at a given instant in time by one of a plurality of light feeder optical fibers, which plurality of optical fibers have been divided and bundled into one or more light receiving channels, whereby light is received in only one of the channels at any particular moment of time, which time correspond to the appearance of the reflected light at a particular optical fiber.
It is therefore a further object of the invention to provide a structure for a light channeling surface and associated light receiving channels.